1. Field of the Invention
The present invention relates to an active matrix display device and a driving method thereof. In addition, the invention relates to an electronic appliance having the display device as a display portion.
2. Description of the Related Art
In order to perform multi-gray scale display using display elements, gray scales are required to be expressed by appropriately controlling the elements by means of a physical value. For example, if a liquid crystal element is used as the display element, gray scales are expressed by applying an analog voltage to the element. On the other hand, if an organic EL element is used as the display element, gray scales are expressed by supplying an analog current to the organic EL element. However, methods utilizing an analog value have limitations on the display accuracy. In particular, in an active matrix display device using organic EL elements which have been actively developed in recent years, analog current values supplied to the EL elements are controlled with TFTs; therefore, variations in the TFT characteristics directly affect the display characteristics.
Meanwhile, a method utilizing the time as a physical value for expressing gray scales is proposed, in which gray scales are expressed using a pulse-width modulation. Specifically, this is a method for expressing gray scales by using the light-emission time of an electro-optical element. In this method, the electro-optical element has only two states of the light-emission and non-light-emission. Therefore, this method does not involve a problem on the display accuracy unlike the case of controlling the light emission state with an analog voltage or an analog current. As for the light-emission time also, if gray scales are expressed by using a discrete value such as an integral multiple of a predetermined unit time, the electro-optical element can be driven without using analog values.
When implementing the aforementioned method, typically, data on a gray scale is expressed with a binary code, and a sub-frame composed of a data writing period and a data holding period is provided for each digit in such a manner that the data holding period for each digit is set to have a proportionate length to a weighted value of the digit. By sequentially displaying each sub-frame, one frame is formed, and gray scales are expressed by using an integral value of the light-emission time in one frame period. This method requires only n data writing operations for expressing a gray scale of 2n; therefore, it is efficient (hereinafter, this method is called a digital time gray scale method).
Meanwhile, in an active matrix display device, the whole pixels for forming one image screen are divided into multiple groups, and data writing is performed in parallel for the pixels within one group while data writing is sequentially performed for each group in a time division manner. Conventionally, a common scan line is provided for each pixel row and a common data line is provided for each pixel column so that data is written to the pixels in the same row in parallel in a time division manner, that is a line sequential driving method has been adopted.
[Non-patent Document 1] K. Inukai, et al., “4.0-in. TFT-OLED Displays and a Novel Digital Driving Method”, SID Dig. Tech. Papers, pp. 924-927, May 2000
[Patent Document 1] Japanese Patent Laid-Open No. 2002-32057
[Patent Document 2] Japanese Patent Laid-Open No. 2002-175047
[Patent Document 3] Japanese Patent No. 2853998
[Patent Document 4] PCT WO01/52229
[Patent Document 5] Japanese Patent Laid-Open No. 2003-208126
There is a case that a contradiction occurs when combining a digital time gray scale method and a line sequential driving method in an active matrix display device. That is, since both of the digital time gray scale method and the line sequential driving method are performed in a time division manner by utilizing the time axis, there is a case that it is difficult to keep the predetermined writing order without contradiction. More specifically, if the time required for the data writing to the whole pixel rows is longer than a data holding period of a sub-frame corresponding to a low-order digit which is weighted into a small value (namely, the shortest sub-frame period), it is required that multiple pixel rows be scanned simultaneously.
As a means for solving the problem, such a method is proposed that a reset function be provided in addition to the normal scan and writing functions, and a non-light-emission period be provided after a sub-frame having the shortest data holding period so that the next sub-frame is started after completing the data writing operation in the present sub-frame (see Non-patent Document 1). However, this method has a problem in that an additional circuit is required in the pixel, which decreases the aperture ratio. Patent Document 1 and Patent Document 2 disclose alternative methods for achieving the aforementioned reset operation without providing an additional circuit in the pixel; however, both have a problem in that the ratio of the light-emission time relatively to a frame period (duty ratio) is decreased by providing the non-light-emission period.
Meanwhile, Patent Document 3 and Patent Document 4 disclose methods for maintaining the duty ratio high without performing the reset operation. In these methods, the time required for data writing to one pixel row is divided into multiple sub-periods, and data writing is performed to different pixel rows in each sub-period, thereby multiple pixel rows are simultaneously scanned spuriously. However, such methods have a problem in that the power consumption is increased since the driving frequency of data lines is increased. In addition, as disclosed in Patent Document 5, there is another problem in that the ratio of the data holding period for each digit cannot be exactly equal to the ratio of a weighted value of each digit.